Dc-dc regulated inverter employing pulse-width modulation with a constant volt-second sensing transformer

ABSTRACT

A static DC-DC inverter employs an output transformer utilizing core materials of different permeability characteristics. The transformer establishes a constant volt-second product as concerns the output waveform and establishes self-regulation on an open loop basis with inherent preventatives as concerns excessive output current spike generation.

[451v May 2, 1972 [56] References Cited UNITED STATES PATENTS COMMONEPOXY ENCAPSULATION PTNTEDIIII 2 |912 SHEET 1 QI 3v FIG.

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CON STANT VOLTAGE (D4 f o LINE VOLTAGE 52| RATIO MAXIMUMy O cu AGENTDC-DC REGULATED INVERTER EMPLOYING PULSE- WIDTH MODULATION WITH ACONSTANT VOLT- SECOND SENSING TRANSFORMER This invention relatesgenerally to power supplies and more particularly to an improved DC-DCstatic inverter which is inherently self-regulating on an open loopbasis and in which the generation of excessive output current spikes dueto core saturation is obviated.

Simple and inexpensive self-excited inverters could be .used in moreapplications if it were not for the excessive current spikes atsaturation. Circuitries operating on a balanced basis and employingsaturable cores areA subject to unbalance should the driving oscillatordrift away from a central balanced condition. 4The resulting unbalanceleads to saturation of the square wave high-p, core generally employedand thus the generation of high input current spikes.

Means have been employed in the art to in one way or another sensesaturation of the core and employ means to cut off the driving currentwaveform as core saturation occurs or preferably prior to coresaturation. For example, means are known to employ an auxiliary outputcore having a higher permeability which in response to any given drivingvoltage waveform inherently saturates prior to the main power core.Means sensing the saturation of the auxiliary core may then be employedto cut off the driving voltage waveform and thus prevent the output corefrom reaching saturation during'successive half cycles.

The present invention relates to a static inverter of the general typeemploying a pair of transistor switching elements to control theapplication of input power to a saturable core v output member. Thepresent invention utilizes core materials of different permeabilitycharacteristics to provide, in effect, a sensing of a time at whichmagnetizing current to the main core should be cut off to preventsaturation thereof. The present invention employs a constant volt-secondsensing transformer with a core member constructed of discreetlydifferent magnetic materials having a high permeability ratio. Inaccordance with' the present invention the constant. voltsecond sensingtransformer establishes a constant volt-second product as concerns theoutput waveform which is an inherent design constant of the transformeritself, thus no sensing of the secondary output is required forregulation.

A further object of the present invention is the provision of a staticinverter power supply which provides good load regulation under openloop characteristics with only the output winding andsubsequentrectifying diodes and filtering choke members being factors ofload regulation.

Yet another object of the present invention is the provision of a staticinverter wherein detrimental primary current spikes stemming from coresaturation are limited since a secondary magnetic path is alwayspresent.

A still further object of the present invention is the provision of astatic inverter which permits a reasonable amount of DC unbalance andambient temperature variation with no detrimental effects on aninherently open loop regulated output.

The present invention is featured in the provision of a static invertercircuitry wherein the saturable core member is cornprised of atransformer having two cores so constructed as to provide a constantvolt-second output in a manner requiring no sensing of secondary output,thus establishing good line regulation on an open loop basis.

These and other objects and features of the present invention willbecome apparent upon reading the following description with reference tothe accompanying drawings in which;

FIG. l is a diagrammatic representation illustrating constantvolt-second waveforms;

FIG. 2 is a schematic representation of a constant voltsecond sensingtransformer in accordance with the present invention;

FIG. 3 shows typical, dynamic permeability curves of a twocoretransformer as illustrated in FIG. 2;

FIG. 4 is a schematic diagram of an embodiment of a constant volt-secondsensing transformer employed in a regulated inverter in accordance withthe present invention;

FIGS. 5 and 6 represent typical operational waveforms of the inverterembodiment of FIG. 4; and

FIGS. 7 and 8 illustrate dynamic permeability curves of a typicaltwo-core transformer as depicted in FIG. 2 illustrating At variationwith core temperature variation.

The present invention might best be contemplated from a consideration ofconstant volt-.second waveforms as depicted in FIG. l. Waveforms (a) and(b) of FIG. 1 depict two square wave voltage waveforms having the samerepetition rate but a decidedly different pulse duration and amplitude.Waveform (a) depicts, during the time le, a square waveform of magnitudeEl with `a duration t1. The volt-second product of waveform (a) is,therefore, E111. Waveform (b) depicts a square waveform of magnitude E2with a shorter time duration t2. The magnitude E2 of waveform (b)exceeds the magnitude E1 of waveform (a). The average voltage for thewaveform depicted in (a) may be expressed as Effi/1C (l) The averagevoltage for waveform (b) of FIG. l may be expressed as E2't2/tc (2)Expressions (l) and (2) illustrate that the average voltage of each ofwaveforms (a) and (b) will be a constant if during a fixed time period,`tc a constant volt-second product is induced. The ratio of thevolt-second product factors E and l) has no influence on the averagevoltage. (It does influence the RMS voltage.) The volt-second areadepicted in waveforms (a) and (b) of FIG. l illustrates waveforms havingthe same average value. The voltage induced in a square wave transformermay be expressed as l E= I`--Bvl-N-4-10s (3) By rewriting expression`(3), substituting l/l for F, the following expression results.

f E't B'A'N'4'l09 (F= l/t) tin seconds (4) A volt-second product appearson the left hand side of the equation (4) while, on the right side ofthe equation are the cross sectional core areaA and the number of turnsN, which are designed constants for a particular transformer. lf theinduction B could also be made constant, then it is seen that thevolt-second product E't on the left side of equation (4) would be adesign constant.

In accordance with the present invention this relationship is utilizedto advantage in an application employing a constant volt-second squarewave transformer in a saturable iron oscillator circuitry, since theinduction B in such applications is always B, the saturation induction,which is a constant of the type of iron employed in the core. Thus, in asaturable core oscillator application the transformer iron is drivenfrom -Blr to +B, or vice versa. Since the induction B in such anenvironment is a design constant, a constant volt-second product wouldbe created in this time interval and a constant average voltage outputachieved.

The present invention utilizes the above discussed considerations in aconstant volt-second sensing transformer employed as an outputtransformer in a static inverter circuitry.

A constant volt-second sensing transformer based on the above discussedconsiderations is depicted diagrammatically in FIG. 2. The transformeris comprised of first and second core members 10 and l1. Core memberl0vmight be comprised of iron and have a permeability u1 while core 11is cornprised of ferrite having a considerably lower permeabilityl u2.Each of the core members l0 and l1 has an identical cross sectionalarea. A common center leg is comprised of equal areas of both the ironand ferrite cores.

For the transformer depicted in FIG. 2, it can be stated that 4,1 'l' 2@or (5) (A: 'l' A2 Amr) The following relationships are assumedNPR=N1=N2=N3 el e2 A, A2 A i With the above assumption the term withinthe bracket of expression (13) is a constant, K, and for the transformerde- E=/12=1+fL./p,2v 18) Equation (17) shows that in the transformer ofFIG. 2, the ratio of the voltages in the two outside legs isproportional to the ratio of the permeabilities of the materials fromwhich the legs are constructed. The permeability of a ferro-magneticmaterial is a function of magnetic flux qb, but in the consideration ofa saturable square wave transformer, the flux d: is proportional to thetime t defining one half-cycle of the square wave. It can be shown,therefore, that the slope of p. with respect to magnetic flux Q over onehalf-cycle of a square wave is the same as the slope of ,u with respectto Kt, where K is a constant dimensionally equal to B/t.

The slopes (variation of magnitude with time) of the permeabilities y.,and ,u.2 associated with the two-core materials obviously differ asdepicted in the dynamic permeability curves of FIG. 3 which illustrate acurve 16 depicting the dynamic permeability function as concerns p., ofthe iron core leg l0 and the dynamic permeability of curve 17 depictingthe variation of f1.2 of the ferrite core within a half-cycle squarewave time period.

If the slopes of ,al and u2 differ, the individual slopes of the inducedvoltages El and E2 in the respective legs windings differ, and would notbe half-cycle square waves, but the sum of the two slopes would besquare waves because El E2 E2 E PR. With reference to expression (18)above, it is seen that E2 approaches E2 when ,um the permeability of theiron core, is very small in proportion to u2. FIG. 3 illustrates that ata point 18 approximately 90 percent through the duration of onehalfcycle square wave time, the higher permeability iron core (pq)saturates and the permeability thereof decreases rapidly at saturation.Thus, at the time of saturation of the iron core, the condition that u,is small in proportion to u2 is met, and as above discussed, at thistime E2 approaches E3 in magnitude. This concept is of paramountimportance in considering the application of the above discussedconstant volt-second square wave transformer in an inverter arrangementin accordance with the present invention.

FIG. 4 shows the embodiment of a driven inverter system in accordancewith the present system which employs the aforedescribed constantvolt-second transformer in a circuitry immune from high voltage, hightransient input voltages which exhibits excellent line regulation over awide input line voltage magnitude range under open loop operatingconditions and inherently eliminates detrimental primary current spikesdue to primary core saturation.

The circuitry of FIG. 4 may generally be defined as comprised of amaster oscillator 23 followed by a power amplifier driving a constantvolt-second transformer the constant average output of which is appliedto a succeeding rectifier circuitry. The master oscillator 23 employs asaturable iron core 25 and, by conventional implementation of switchingtransistor pair 24, generates a constant frequency square wave outputwaveform. The oscillator 23 might thus be a conventional self-driven DCto AC converter circuitry. A constant voltage source 21 is employed forthe master oscillator 23 in order to assure a constant output frequency.

The power amplifier circuitry, driven by the square wave output fromoscillator 23 functions, by employing the above described constantvolt-second transformer, a constant voltsecond output to the rectifier42 that is, supplies a constant average output signal with widevariations in line voltage magnitude.

The power amplifier employs switching transistors 26 and 27 under thecontrol of the constant frequency square wave signal from masteroscillator 23 to appropriately switch a line voltage input on terminals19 and 20 to the constant voltsecond transformer 40 from which an outputis applied to the succeeding rectifier circuitry 4l.

ln the arrangement to be described, the line voltage might vary over awide range (for example 5:1) in DC magnitude and a constant outputvoltage will be developed by the rectifier filter 41-42 for applicationto the load 43. The regulation, as will be described, is accomplishedinherently with no output feedback and thus operates in an open loopmode to maintain excellent line regulation.

With reference to FIG. 4, the secondary winding 30 of the transformer ofdrive oscillator 23 is provided with a center tap 34 connected through aresistor 35 in common to the emitter electrodes of power amplifierswitching transistors 26 and 27. The base electrodes of switchingtransistors 26 and 27 are respectively connected to symmetricalsecondary winding taps 36 and 33. The end terminals 31 and 32 of thesecondary winding 30 of the oscillator transformer are connected to therespective anode electrodes of first and second silicon controlledrectifiers 28 and 29. The cathodes of rectifiers 28 and 29 are connectedto the respective emitters of switching transistors 26 and 27. The gateelectrodes of silicon controlled rectifiers 28 and 29 are respectivelyconnected through resistors 37 and 38 to the common emitter connectionof the switching transistors 26 and 27. The collector electrodes ofswitching transistors 26 and 27 are connected to respective ends of theprimary winding l5 on transformer 40, which is wound common to both theiron and ferrite core members. Terminal 19 of the DC line voltage inputis connected to the center tap of the primary winding 15 of transformer40. The other terminal 20 of the DC line voltage input is connected tothe emitters of switching transistors 26 and 27 and to the center tap ofa sensing winding 13 on the ferrite core 11 of transformer 40.Sensing'voltages are 'developed across resistors 44 and 45 which shuntthe respective halves of centertapped-sensing winding 13. v f n Oneterminal of resistor 44 is connected through a diode member 46 and areverse-polarized zener diode 48 to the control electrode of siliconcontrolled rectifier 28. The end terminal of resistor 45 is connectedthrough a further diode member 47 and reverse-polarized Zener diode 49to the control electrode of the other silicon controlled rectifiermember 29. An output winding 14 is wound in common to both the iron andferrite core members of transformer 40 and connected in a full-waverectifying arrangement with rectifying diode-pair 41, and LC filter 42.A load 43 is connected to the filter output.

The operation of the inverter circuitry of FIG. 4 is based on theconstant volt-second characteristics of the power transformer 40. Asdepicted in FIG. 3, the iron core member with permeability y2 saturatesprior to the end of a half-cycle of the switching control waveform fromoscillator 23. The ferrite core member 1 1 because of its lowpermeability does not saturate in this time interval. For example,nickel-iron might be employed having a permeability between 50,000 and300,000 along with a ferrite core having a permeability between 2,0003,000. High- ,u nickel-iron saturates at approximately 0.2 oersted whileferrite saturates at approximately 8.0 oersted. Winding 13 on theferrite core 11 operates as a sensing winding to produce sense voltagesE2 during successive half-cycles of the square wave oscillator waveformapplied to the power amplifier.

ln operation, and neglecting for the moment the function of the sensingvoltages E2 and their relationship with silicon controlled rectifiermembers 28 and 29, the power amplifier,

under control of the constant frequency square wave signal from themaster oscillator 23, switches the line voltage from terminals 19 and 20to the primary winding 15 of the power amplifier transformer. During onehalf-cycle switching transistor 26 is provided with base drive so as tobe conductive and provide a low impedance path between one end ofprimary winding l5 of transformer 40 and terminal 20 of the line voltagesource. In a successive half-cycle, switching transistor 29 is providedwith base drive and conducts to provide a low impedance path between theother end of winding l5 and line l terminal 20. Line voltage terminal 19is applied to the center tap of the primary winding l5 and thus linevoltage is successively applied to the respective sides of the primarywinding in a full-wave manner. The line voltage is, therefore, appliedto the power amplifier transformer at the initiation of successivehalf-cycles of the oscillator 23 waveform.

Previous discussion with reference to FIG. 3 indicated that transformer40 is so designed that iron core 10, having permeability pq, saturatesat about 90 percent through the half-cycle of the oscillator waveformwhen excited with minimum input voltage. Because of the highpermeability ratio between the two core members, while the iron core upto its point of saturation was supporting most of the primary voltage,upon saturation of the iron core, all of the input voltage istransferred to the ferrite core.

The sense winding 13 on the power amplifier transformer is wound aboutthe ferrite core member l1 only, and produces the sense voltage E2. Withreference to the waveforms of FIG. 5, the voltage E3 developed onsecondary winding 14 of the power amplifier transformer is the sum (ElE2) of the voltages induced by the flux in both the iron and ferritecores. Thus the oscillator waveform Em at time to produces an output E:lprior to saturation of the iron at time tl which, due to the ratio ofpermeabilities of the iron and the ferrite, results from flux in theiron core which is supporting most of the primary voltage. Upon ironcore saturation at time 1 the input voltage is transferred to theferrite core and this sharply increases the sense voltage E2 for theensuing portion of the oscillator half-cycle, that is, the time tl.- t2.Waveforms of FIG. 5 depictthe inverter operating on a 100 percent dutycycle without regulation, that is, with the sense voltage E2 not beingused for firing the silicon controlled rectifiers. FIG. 5 illustratesthat no current spike appears in the output E3 from the power amplifiertransformer since, upon saturation of the iron core supplying E theunsaturated ferrite core supports the magnetizing current.

From FIG. 3 it can be assumed that the permeability p2 of the ferritecore is constant over the time period where the permeability y., of thenickel-iron core rapidly decreases. This indicates that the slope of thesensing voltage E2 at this instant is a function of only ,u.1, thepermeability of the iron core.

Equation l 8) clearly illustrates this.

At the time of saturation of the iron core the input voltage istransferred to the ferrite core, increasing the sense voltage E2 to thepoint where it overcomes the threshold of one of the zener diodes 48 and49 to fire the associated silicon controlled rectifier. The conductingone of the silicon controlled rectifiers 28 and 29 during a particularhalf-cycle of the oscillator waveform, raises the emitter voltage of theconducting power transistor above the base drive voltage thereof,effecting shut off of the conducting power transistor which in turnshuts off the excitation of the power transformer 40. The siliconcontrolled rectifier thus forced into conduction remains in itsconducting condition until the oscillator 23 completes it half-cycle.This period of time is a dead time period during which no power is beingapplied to the output.

In the ensuing half-cycle, the iron core 10 of the power amplifiertransformer is driven into saturation in the opposite direction untilthe sense voltage E2 again rises to a sufflciently high potential toovercome the threshold of the other one of zener diodes 48 and 49 andfire its associated silicon controlled rectifier. Thus in everyhalf-cycle of the waveform of oscillator 23, a constant volt-secondproduct is produced and a constant average voltage achieved. Should theline voltage rise in magnitude the iron core saturates at aproportionally earlier period of time within each successive'half-cyclesuch that the product of voltage and time remains constant. Since thevoltage Ea on the output winding 14 of the power amplifier transformerhas a constant volt-second product, a constant average voltage isachieved and the subsequently rectified waveform is a constant DCvoltage.

Operational waveforms of the converter embodiment of FIG. 4 underregulating conditions where the sense voltage is applied back to controlthe application of power to the primary of transformer 40 are depictedin FIG. 6.

The above statement that a constant volt-second product is produced anda constant average voltage achieved is, of course, based on theassumption that BSH of the transformer is constant. In practice, BSM isa function of temperature. By using the same type of iron for core l0 ofthe power amplifier transformer as is employed in the core 25 of drivingoscillator 23, the temperature factor is canceled out because thevoltsecond product of the two-core transformer 40 decreases in the sameratio as the frequency of oscillator 23 increases with risingtemperature.

By way of summary of design considerations, the foregoing discussion hasindicated that the two transformer cores have a high permeability ratio.Permeability of the low permeability core for any application should behigh enough to prevent a large current spike when the high permeabilitycore saturates.

High-p. square-loop nickel-iron and square-loop ferrite fulfill theabove requirement. Square-loop materials have a small stored energyrelease after each half-cycle, and thus the voltage spike at the startof each half-cycle may be minimized. This characteristic is of a specialimportance when considering the predetermined sensing voltage E2threshold utilized to fire the silicon controlled rectifiers.

Some DC unbalance in the ferrite core is permissible in the inverterbecause of the high excitation current necessary to saturate the lowp.ferrite.

The high-u iron core should also have a square-loop characteristic. Asabove described, the voltage E2 utilized in firing the siliconcontrolled rectifiers increases to the firing level threshold when thepermeability of the high-1.a. iron decreases to a certain point. It isat this point that the input voltage is transferred to the ferrite core.The discussion herein, for simplicity, has defined this point as beingwhen the permeability of the iron and ferrite cores are equal (,u.l p2).The permeability 2 of the ferrite core varies noticeably withtemperature. This variation in p2 with change in temperature has agreater influence lon the obtained volt-second product when employinground-loop iron than when employing square-loop iron. With reference toFIG. 7, the dynamic permeability curves of a typical round iron (,ul)and a typical square iron (u1) are plotted with that of a typicalferrite material (u2). The variation of the ferrite permeability p2under different core temperature conditions is illustrated as a secondu2 curve with distinctly different slope.

FIG. 8 illustrates an enlarged portion of that circled as View A in FIG.7. It is noted that the variation in time (At round) is greater when around iron core is utilized than the variation in time (At square) whena square iron core is utilized, due to the steeper fall off slope of thesquare iron characteristic. Therefore, for a given variation in theferrite perrneability u2 with temperature, as depicted by the two f1.2curves, the At interval is considerably less when employing square ironin the output transformer core than whenemploying a round iron materialwhich exhibits a mofe gradual slope at saturation.

From a further design standpoint, since the average output voltage is afunction of both the oscillator frequency and of the obtainedvolt-second product, and both in. turn are functions of temperaturebecause of the iron employed in both transformers, the temperature ofboth transformers should be kept the same and to accomplish thisrequirement both transformers might preferably be encased in a heatconducting epoxy and mounted on a common heat sink, as depicted by thecommon epoxy encapsulation 50 of FIG. 4 and common heat sink 51.

The advantages of pulse width modulation with a constant volt-secondsensing transformer may be summarized as follows:

The constant volt-second product is an inherent design constant of thetransformer itself and easy to obtain. No sensing on the secondaryoutput is required.

High transient input voltages do not reflect on the output voltage.

Excellent line regulation is obtainable over input ranges as great asfive to one with open loop operation.

Detrimental primary current spikes through saturation of the iron areeliminated because the induction of the ferrite core is always present.

A reasonable amount of DC unbalance has no detrimental effect onoperation.

. Although the present invention has been described with respect to aparticular embodiment thereof, it is not to be so limited as changesmight be made therein which fall within the scope of the invention asdefined in the appended claims.

I claim:

1. A static inverter comprising a source of direct current line voltage,switching means; output transformer means, said transformer meanscomprising first and second core members, a primary winding wound commonto each of said core members, a sensing winding wound about said secondcore member, an output winding wound common to each of said first andsecond core members, each of said core members having a squarehysteresis characteristic with the permeability of said second corebeing appreciably less than that of said first core; said transformerprimary, output, and sensing windings being comprised of a like numberof turns, said first and second core members defining like mean magneticpath lengths, and said first and second core members having equal crosssectional areas whereby a constant volt-second product is establishedfor the output waveform from said transformer output winding over apredetermined ratio of line voltage source magnitude; said switchingmeans being adapted to periodically apply said line voltage source tosaid transfonner primary winding for a predetermined period of time theduration of which is less than the time between the initiation of saidperiodic applications, said primary winding and said line voltage sourceestablishing a magnetizing current in said primary winding sufficient tosaturate said first core within the time period between successiveapplications of said line source to said primary winding andinsufficient to effect saturation of said second core during this timeperiod, switching control means connected to said sensing winding andresponsive to the voltage induced therein to disable said switchingmeans at the time of saturation of said first core member, and saidswitching signal source comprising a further static inverter employing afurther saturable core output transformer the core of which comprisesthe same material as that of said output transformer first core member.

2. A static inverter as defined in claim 1 wherein said furthersaturable transformer core and said output transformer first core memberare embodied in a common ambient temperature environment.

3. A static inverter comprising a source of direct current line voltageswitching means; output transformer means, said transformer meanscomprising first and second core members, a primary winding wound commonto each of said core members, a sensing winding wound about said secondcore member, an output winding wound common to each` of said first andsecond core members, each of said core members having a squarehysteresis characteristic with the permeability of said second corebeing appreciably less than that of said first core; said primary,output, and sensing windings being comprised of a like number of turns,said first and second core members defining like mean magnetic pathlengths, and said first and second core members havin equal crosssectional areas whereby a constant volt-secon product is established forthe output waveform from said transformer output winding over apredetermined ratio of line voltage source magnitude; said switchingmeans comprising a source of alternating current square wave switchingsignal of predetermined frequency, transistor switching means receivingsaid switching signal and being rendered conductive thereby insynchronism with successive like half-cycle periods of said switchingsignal, said transistor means being serially connected with said linevoltage source and said transformer primary winding and effecting a lowimpedance interconnection therebetween when conductive, the timeduration of half-cycles of said switching signal being such as to effectsaturation of said first core member prior to the termination of saidhalf-cycles; and switching control means comprising means responsive toa predetermined level of the signal induced in said transformer sensingwinding to render said transistor means non-conductive for the ensuingportion of said switching signal half-cycles, said switching coritrolmeans comprising silicon controlled rectifier means responsive to saidpredetermined sense winding signal level to conduct and disable a basedrive interconnection between said transistor means and said switchingsignal source.

4. A static inverter as defined in claim 3 comprising zener diode meansserially connected between said transformer sense winding and gatingmeans associated with said silicon controlled rectifier means.

5. A static inverter as defined in claim 4 wherein said switching meansis embodied as a further static inverter employing a further saturablecore output transformer the core of which comprises the same material asthat of said output transformer first core member.

6. A static inverter as defined in claim 5 wherein said furthersaturable transformer core and said output transformer first core memberare embodied in a common ambient temperature environment.

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1. A static inverter comprising a source of direct current line voltage,switching means; output transformer means, said transformer meanscomprising first and second core members, a primary winding wound commonto each of said core members, a sensing winding wound about said secondcore member, an output winding wound common to each of said first andsecond core members, each of said core members having a squarehysteresis characteristic with the permeability of said second corebeing appreciably less than that of said first core; said transformerprimary, output, and sensing windings being comprised of a like numberof turns, said first and second core members defining like mean magneticpath lengths, and said first and second core members having equal crosssectional areas whereby a constant volt-second product is establishedfor the output waveform from said transformer output winding over apredetermined ratio of line voltage source magnitude; said switchingmeans being adapted to periodically apply said line voltage source tosaid transformer primary winding for a predetermined period of time theduration of which is less than the time between the initiation of saidperiodic applications, said primary winding and said line voltage sourceestablishing a magnetizing current in said primary winding sufficient tosaturate said first core within the time period between successiveapplications of said line source to said primary winding andinsufficient to effect saturation of said second core during this timeperiod, switching control means connected to said sensing winding andresponsive to the voltage induced therein to disable said switchingmeans at the time of saturation of said first core member, and saidswitching signal source comprising a further static inverter employing afurther saturable core output transformer the core of which comprisesthe same material as that of said output transformer first core member.2. A static inverter as defined in claim 1 wherein said furthersaturable transformer core and said output transformer first core memberare embodied in a common ambient temperature environment.
 3. A staticinverter comprising a source of direct current line voltage switchingmeans; output transformer means, said transformer means comprising firstand second core members, a primary winding wound common to each of saidcore members, a sensing winding wound about said second core member, anoutput winding wound common to each of said first and second coremembers, each of said core members having a square hysteresischaracteristic with the permeability of said second core beingappreciably less than that of said first core; said primary, output, andsensing windings being comprised of a like number of turns, said firstand second core members defining like mean magnetic path lengths, andsaid First and second core members having equal cross sectional areaswhereby a constant volt-second product is established for the outputwaveform from said transformer output winding over a predetermined ratioof line voltage source magnitude; said switching means comprising asource of alternating current square wave switching signal ofpredetermined frequency, transistor switching means receiving saidswitching signal and being rendered conductive thereby in synchronismwith successive like half-cycle periods of said switching signal, saidtransistor means being serially connected with said line voltage sourceand said transformer primary winding and effecting a low impedanceinterconnection therebetween when conductive, the time duration ofhalf-cycles of said switching signal being such as to effect saturationof said first core member prior to the termination of said half-cycles;and switching control means comprising means responsive to apredetermined level of the signal induced in said transformer sensingwinding to render said transistor means non-conductive for the ensuingportion of said switching signal half-cycles, said switching controlmeans comprising silicon controlled rectifier means responsive to saidpredetermined sense winding signal level to conduct and disable a basedrive interconnection between said transistor means and said switchingsignal source.
 4. A static inverter as defined in claim 3 comprisingzener diode means serially connected between said transformer sensewinding and gating means associated with said silicon controlledrectifier means.
 5. A static inverter as defined in claim 4 wherein saidswitching means is embodied as a further static inverter employing afurther saturable core output transformer the core of which comprisesthe same material as that of said output transformer first core member.6. A static inverter as defined in claim 5 wherein said furthersaturable transformer core and said output transformer first core memberare embodied in a common ambient temperature environment.